Health risks from radiation exposure may be described in terms of acute effects and long term risks. The extent and severity of acute effects is determined by the type and amount of radiation exposure, and they range from mild and recoverable effects, such as nausea and vomiting, to central nervous system damage and even death. Long term risks include development of cataracts and an increase in the probability of the development of cancer. Results from animal experiments show that the biological damage to the central nervous system which is unique to the high-energy, heavy ions encountered in space is similar to that associated with aging. There is experimental evidence that radiation encountered in space is more effective at causing the type of biological damage that ultimately leads to cancer than gamma or x-rays commonly encountered on Earth.
An increase in cancer risk is the principal concern for astronaut exposure to space radiation
, and it is one risk that persists after landing. This concern arises from the fact that exposure to radiation of sufficient energy causes ionizations of the molecules of living cells. At low doses, such as that which we receive every day from background radiation, cells can repair the damage rapidly. At higher doses, the cells may not be able to repair themselves and they can either be changed permanently or die. Most cells that die are of little consequence,
Radiation interacting with a DNA molecule.
because the body can replace them.
However, cells that change permanently can go on to produce other abnormal cells, which, under the right circumstances, may become cancerous. If the mutation occurs in a sperm or an egg cell, mutation of offspring may result. At very high doses, the cells cannot be replaced fast enough, and the tissues fail to function. In such cases, radiation sickness, or even death, can result.
There are two major ways that radiation can damage cells:
- The water in the organism (e.g., a person's body) absorbs a large portion of the radiation and becomes ionized to form highly reactive, water-derived radicals. The free radicals then react with DNA molecules causing the breaking of chemical bonds or oxidation.
- The radiation collides with the DNA molecule directly.
In either case, the DNA molecule breaks. A DNA molecule is composed of two strands of nitrogen-containing molecules which link together to form bonds with each other similar to the rungs in a ladder. These linked strands then twist around one another to form a helical structure that carries genetic information. Breaks can occur at either or both of the strands, but interactions that result in breaks to both strands are believed to be more biologically significant. With single-strand breaks, the cell can usually repair itself and resume normal function. This repair process can happen because the double-stranded nature of the DNA molecule allows the undamaged strand to serve as a template for the repair of the broken one. With double-strand breaks, however, the repair is more difficult, and the cells may either be changed permanently or die.
Astronauts have been classified as radiation workers and, monitoring their radiation exposure has been a key requirement for spaceflight since Project Mercury. Terrestrial radiation guidelines are considered too restrictive for space activities. Therefore, NASA has adopted the recommendations of the National Council on Radiation Protection (NCRP) for spaceflight activities. These limits, which are considerably higher than those for terrestrial radiation workers (5 rem per year), are detailed in the table below.